Biomechanics and Medicine in Swimming XI

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Table 1. Mean ± SD values of the 15 linear biomechanical parameters studied for backstroke start variants with feet immersed and emerged (BSFI and BSFE, respectively). BSFI BSFE p ST (s) * 0.93 ± 0.08 0.98 ± 0.11 0.01 HOT (s) * 0.55 ± 0.04 0.59 ± 0.08 0.03 TOT (s) * 0.23 ± 0.05 0.27 ± 0.04
BiomechanicsandmedicineinswimmingXi dIscussIon The results showed that a higher foot position of BSFE produces a higher horizontal wall approximation of the CM in the starting position. In the same way, BSFE presented a higher HOT. Differences may certainly be attributed to the foot segment constraint in the starting position, which seems to imply a more complex lower and upper limb movement sequence to release handgrip contact at BSFE. Previous values published without these constraints presented values between 0.42 and 0.52 s (Kruger et al., 2006). Differences may be attributed to the higher competitive level of the swimmers of the referred study. In accordance with the analysing movements reported in previous studies such as vertical, horizontal, drop or rebound jumps (Bobbert and Van IngenSchenau, 1988; Rodacki and Fowler, 2001), during the backstroke start (both variants), the CM is initially accelerated by the extension of the hip joint, although BSFE registered lower time to reach joint peak angular velocity. We can speculate that the more tucked position of the BSFE may imply a higher pre-stretching of the subsequent propulsive muscles. In this situation, the force production capacity of these muscles can be maximized if the time between the stretch and shortening actions, and their loading characteristics respect particular limits. This has yet, however, to be verified. At BSFI condition a higher knee joint angular displacement was observed, which seems to contribute to a higher DxCM-HO. The present results confirmed the hypothesis that the BSFI start condition might result in a different resultant vector angle with a more pronounced vertical component as noted by the higher DyCM-TO. As Counsilman (1971) previously stated, few swimmers can perform successfully the BSFE due to the longer training required to increase the vertical component at the instant of take-off. Temporal analysis also revealed that BSFE obtained higher TOT. It may be due to the specific placement of the feet above water level, which seems to imply that the body mass was accelerated through a greater distance after the start signal until the take-off, which explains the higher impulse. The analysis of segmental co-ordination indicated that BSFI showed greater angular velocity of the knee joint at the beginning of the take-off phase; while, the BSFE condition presented a greater time of the knee joint maximal extension. Additionally, BSFI imposed a lower time to reach ankle joint maximal extension, at the end of TO phase. The paired sample T-test revealed higher FLT and X1 for the BSFI. In a previous study, Miller et al. (1984) found values of 0.11 ± 0.06 s (100 m events) and 0.11 ± 0.05 s (200 m events) for FLT during the backstroke start without foot position specified (but assumed to be BSFI), values that suggested being lower than those found in our study for both variants. Additionally, these authors found similar values to ours for X1, 2.77 ± 0.12 m (100 m) and 2.78 ± 0.12 m (200 m). Nevertheless, differences may be attributed to the end point measurement of the flight analysis, which was established by Miller et al. (1984) as distance to the hand contact with the water instead of the distance to the CM at the hand touch. conclusIons As a performance parameter, the total time spent during the start was lower for BSFI than BSFE, allowing the conclusion that the first, being faster than the second should be preferred for competitive use. This observed superiority of the BSFI may be at least partially justified by the higher flight time (FLT) and reach of the centre of mass (X1). These findings seem to confirm the hypothesis that a lower foot position can determine the CM water reach by constraining the orientation of the resultant wall reaction vector. Inter-segmental coordinative analysis of the lower limbs showed that the relative time at which the knee and ankle peak joint angular velocity occurs might be an important variable to characterize the co-ordination pattern, and to explain the performance capacity of the BSFI. It is recommended that coaches begin monitoring the backstroke start variation strategies to improve inter-segmental coordination, which can be the determining factor of success of the start. 66 reFerences Bobbert, M. & Van Ingen Schnenau, G. (1988). Co-ordination in vertical jumping. J Biomech, 21, 249-62. Counsilman, J. E. (1971). La natación: ciencia y técnica para la preparación de campeones. Englewood Cliffs, New Jersey: Prentice-Hall, Inc. De Leva, P. (1996). Adjustments to Zatsiorsky-Seluyanov’s segment inertia parameters. J Biomech, 29(9), 1223-30. Hohmann, A., Fehr, U., Kirsten, R. & Krüger, T. (2006). Emg-model of backstroke start technique. Port J Sport Sci, 6(1), 37-40. Krüger, T., Hohmann, A., Kirsten, R. & Wick, D. (2006). Kinematics and dynamics of backstroke start. Port J Sport Sci, 6(1), 58-60. Lyttle, A. & Benjanuvatra, N. (2004). Start right: biomechanical review of dive start performance [electronic version]. Available at Coaches Information Website: http://www.coachesinfo.com/category/swimming/321/ Miller, D., Hay, J. & Wilson, B. (1984). Starting technique of elite swimmers. J Sports Sci, 2, 21-27. Rodacki, A. & Fowler, N. (2001). Intermuscular coordination during pendulum rebound exercises. J Sports Sci, 19, 411-425. Roesler, H. (1997). Development of underwater platform for force and force moment measurement in all coordinate axes for biomechanical applications. PhD thesis, Federal University of Rio Grande do Sul, Porto Alegre. Van Ingen Schenau, G. (1989). From rotation to translation: constraints on multi-joint movements and the unique action of bi-articular muscles. H Mov Sci, 8, 301-377. Vilas-Boas, J. P., Cunha, P., Figueiras, T., Ferreira, M. & Duarte, J. (1997). Movement analysis in simultaneously swimming techniques. Kolner Swimmsporttage–Symposiums. Bericht, Germany: Sport Fahnemann Verlag. Vilas-Boas, J. P., Cruz, J., Sousa, F., Conceição, F., Fernandes, R. & Carvalho, J. (2003). Biomechanical analysis of ventral swimming starts: Comparison of the Grab Start with Two Track-Start Techniques. Biomechanics and medicine in swimming IXth. Saint-Étienne, France: Université de Saint-Etienne. Welcher, R., Hinrichs, R. & George, T. (2008). Front-or-rear weighted track start or grab start: which is the best for female swimmers? Sports Biomech, 7(1), 100-13.
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Pahlen Norge AS Pahlen Norge AS Cha
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